It has heretofore been proposed to employ so-called time domain reflectometry techniques to detect level of material in a storage vessel. In general, this technique involves placement of a conductive transmission line probe in the vessel at an orientation to be contacted by material in the vessel. A microwave pulse of short duration is launched onto the transmission line probe, typically in a downward direction through air toward the material surface. When this pulse encounters an electrical discontinuity, such as the change in dielectric constant at the interface between the air and material, a portion of the pulse energy is reflected back along the transmission line probe to detection electronics. Time-delayed gating techniques are employed in a process referred to as equivalent time sampling to locate the position of the reflecting discontinuity along the transmission line probe, and thereby to determine level of the material surface with respect to the probe.
Although the material level sensing technique so described has overcome problems and difficulties theretofore extant in the art, further improvements remain desirable. For example, a problem with time domain reflectometry level measurement techniques lies in an ambiguity in level measurement at the free end of the transmission line probe. Specifically, it has been found that a phase reversal takes place in the signal reflected from the free end of the transmission line probe when the material just contacts the free end of the probe. This phase reversal causes the detection electronics to indicate a material level greater than actual material level. On the other hand, there is an opposite phase reversal in the reflected signal when the material just moves out of contact with the probe. Consequently, there is a zone of ambiguity, generally corresponding to transmission pulse length, at material levels near the end of the probe. In very short vessels, this zone of ambiguity can represent a significant percentage of overall height.
Numerous systems and techniques have heretofore been proposed for continuous measurement of material level within a storage vessel between preset or predetermined operating limits. For example, it is conventional practice to provide a material level-indicating output over a continuous current range of four to twenty milliamps corresponding to a material level range within the vessel between empty and full conditions. Insofar as applicant is aware, all of such systems require that the probe or other level-responsive means be placed within the vessel, and that the material be raised and lowered within the vessel between levels corresponding to the full and empty conditions in order to calibrate the range limits of the detection electronics.
It is a general object of the present invention to provide a material level measurement system and method, employing time domain reflectometry techniques or technology, with facility for calibration outside of the material vessel, such as at the time of manufacture or immediately prior to installation into a vessel. A more specific object of the present invention is to provide a system and method of the described character in which zero and span calibration, corresponding to the lower and upper ends of the continuous material level measurement range, can be calibrated by relatively unskilled personnel prior to installation in the vessel. A further specific object of the present invention is to provide a material level measurement system that employs time domain retlectometry techniques in which the transmission line probe is constructed to eliminate material level measurement ambiguity at the free end of the probe.